Novolac alkylphenol resins may be obtained by reacting alkylphenols such as para-tertiary butylphenol (PTBP) and para-tertiary octylphenol (PTOP) with aldehydes, especially formaldehyde, in the presence of acid catalysts. See Knop et al., “Chemistry and application of phenolic resins,” Polymer/properties and Application, ed. Springer-Verlag Berlin Heidelberg New York, p. 10-27 (1979), which gives an overview of the different aldehydes and alkylphenol monomers used in the novolac alkylphenol phenolic resins.
It is known that by varying the molar ratio (MR) of aldehydes(s)/alkylphenol(s), it is possible to prepare resins having different average molecular masses. However, due to the chemical nature of the reaction, a fraction of free phenolic monomers remains after the formaldehyde has been consumed at the end of the reaction. An estimation of the fraction of free phenols has been calculated by considering the formaldehyde/phenol as a statistical polycondensation. See Borrajo et al., Polymer, Vol. 23, February, p. 263-266 (1982). Borrajo et al. discuss that, according to Stokemayer's distribution, a reaction carried out with MR 0.7 yields a rate of residual phenol of 11.6%, a MR of 0.8 yields 6.5%, and a MR of 0.9 yields 3%; the calculation also describes MRs of 9, 4 and 1%, respectively, for a dysfunctional paraalkylphenol. Typically, the molar ratio used for production of commercial novolac alkylphenol resins varies from 0.7 to 0.9.
It is desirable to reduce this rate of residual phenolic monomer(s) for environmental reasons, on one hand, and to diminish the nature of dangers of resins. In fact, alkylphenols like PTOP are the object of environmental and toxicology studies (risk assessment studies) to limit their use in the free monomer form in novolac alkylphenol resins. The decrease in the free alkylphenol rate also permits a simpler manipulation of resins by decreasing the vapor emissions of alkylphenolic monomers in factories or in atmospheric discharges when they are used, in particular rubber applications when vulcanization is carried out at elevated temperature.
The commercial novolac alkylphenol resins used as tackifying resins in the formulation of rubber generally have a softening point temperature between 85 and 105° C. Since the usage temperature are dictated by complex components of the rubber-based formulations and cannot be modified easily without changing the reactivity of the system: the alkylphenolic resins in this softening point temperature range are suitable for allowing the dissolution, the fusion and the dispersion of the resin during its mixing step with the other components of the rubber formulation and at the time of the vulcanizing step.
Wolny et al., Kautsuch Gummi Kunstoffe [Rubber, Plastics] 37:7, p. 601-603 (1984) describes novolac resins from the condensation of the PTOP and formaldehyde that have a temperature close to 100° C. and have a residual free alkylphenol rate of about 4.5%. The MR, however, is about 0.8 to 0.9, and the resins have a softening point value of 85-110° C. A rate of free residual PTOP less than 1% could not be obtained without having a MR greater than 0.9 with a softening point temperature of around 120° C. For a novolac PTOP/formol resin with MR 0.96, the rate of free PTOP is less than 1% and the softening point temperature is elevated (around 130-140° C.). In addition to the elevated temperatures (around 180° C.), the synthesis is delicate because of the significant viscosity of the resin. For resins with PTBP having high softening point temperatures (on the order of 120-130° C.) and a MR on the order of 0.8, the residual PTBP rate is observed to be around 2 to 3%.
The free residual monomer or monomers can be distilled at the end of the resin synthesis to decrease the rate of free alkylphenols. However, this conventional method presents several disadvantages: first, the distillation of the free monomer or monomers represents a not insignificant loss of primary material and requires recycling or treatment of the distilled alkylphenol; second, it is difficult to implement at the industrial level for the alkylphenols like PTOP that have very high boiling points, which involves a very powerful vacuum and elevated temperatures in the reactor in order. Besides that, alkylphenols that are solid compounds, like PTBP and PTOP, crystallize in the piping which requires reheating if plugs are to be prevented.
U.S. Pat. No. 6,326,453 and EP 1,108,734 A1 teach that the reduction in the rate of free phenol in novolac (alkyl)phenol resins can also be obtained by the use of an organophosphonic catalyst. However, a significant amount of catalyst is used. To arrive at a rate of residual free phenol less than 1%, it is necessary to use 60% by weight of organophosphonic catalyst, based on the weight of the phenol, and for a rate of residual phenol of around 2%, it is necessary to use 10% by weight of catalyst. These references also indicated that as the rate of catalyst becomes lower, with 0.1 mol-% in comparison to the phenol, the reaction becomes ineffective.
JP11-349.655 describes the preparation of novolac phenol-formol resins in a methanol solution under supercritical conditions of 15 MPa/250° C. to obtain a rate of free phenol of 1%. This type of procedure is very difficult to implement on the industrial level because of the elevated pressures necessary, as it is preferable to work at pressures close to atmospheric pressure.
The addition of urea is recommended by Li Ziqiang, Mining & Metallurgy, Vol. 5, No. 1, p. 24-27 (March 1996) (CAS 125:115938). According to the Ziqiang, this makes it possible to decrease the free phenol from 18% (without urea) to 5%. But this is still too high. Besides that, the introduction of urea into the product involves the risk of deteriorating the stability of the phenolic resin and thus negatively effecting the final properties of the resulting rubber formulations.
CS 238.995 (CAS 108:57118) describes the preparation of a mixture made up of phenol, formaldehyde (MR 0.97), and stearic acid (3.5% by weight based on the phenol) in the presence of various additives or batches. The resin obtained after reaction of the mixture has a rate of free phenol of 5%.
U.S. Pat. Nos. 2,506,903 and 2,506,904 teach the preparation of greases (which have a softening point value that is close to or less than the ambient temperature) obtained by esterification of a novolac formophenolic resin with fatty acids, the formol/alkylphenol ratio MR described in the application varies from 1 to 2. The preparation process consists either of (a) esterifying the alkyl phenol with the fatty acid near 100° C., which is reacted with formaldehyde at temperatures up to 250° C., or (b) preparing the formaldehyde/alkylphenol resin at a temperature of 100-150° C., then to adding fatty acid to carry out the esterification reaction at a temperature of 200° C. The rate of fatty acid necessary to obtain a product in the form of grease is on the order of 100% of the alkylphenol mass used.
JP09-003.384 (CA/126:187493) describes the modification by esterification of a formophenolic resin of the resol type (prepared with a base catalysis) at a temperature greater than 200° C., typically between 200 and 260° C., with a fatty acid in the presence of rosin or one of its derivatives. The rate of residual phenolic monomer is less than or equal to 1%. This modified resin being used for the manufacture of printing ink.
Accordingly, no satisfying technical solution exists for decreasing the rate of residual alkylphenol(s) in the novolac alkylphenol resins below 2% while maintaining a softening point temperature between 85 and 105° C., so that the resins can be used as tackifying or reinforcing resins in rubber-based formulations. This invention answers that need.